This invention relates to method and apparatus for hydroforming metal parts and more particularly to the seal units employed in the hydroforming which are operable to compress the parts between their ends to prevent or minimize thinning of the parts during their hydroforming.
As is well known in the hydroforming art, like devices commonly referred to as xe2x80x9cseal unitsxe2x80x9d are located at opposite ends of a tubular metal part to be hydroformed. The seal units are then simultaneously advanced by separate hydraulically operated actuators, also called feed mechanisms, to sealing dock with the ends of the tubular part. And this docking of the seal units may be either before or after the part is fully enclosed in a hydroforming die cavity. The thus sealed tubular part is then filled with a hydroforming fluid by a passage through the seal units while the part is fully enclosed in the die cavity and this fluid is then pressurized sufficiently to force the tubular part to stretch outward and conform to the die cavity. Examples of such prior method and apparatus are disclosed in U.S. Patents 5,233,854; 5,233,856 and 5,321,964 assigned to the assignee of this invention.
While such prior method and apparatus have proven generally satisfactory, it is commonly known that the stretching of the metal in the hydroforming of the part causes its wall to thin and thus weaken. And this can present a strength problem depending on the thickness or gage of the tubular part and its intended usage. In any event, it is generally desirable regardless of the end use of the hydroformed part to make the best use of the part""s material provided there is not some overriding factor. For example, in attempting to solve this wall-thinning problem, there may be a substantial cost penalty involved or some other impracticality such as limited space in the hydroforming apparatus for some form of means to compensate for this problem.
One known form of apparatus for preventing or minimizing wall thinning of a hydroformed part which is not provided for in the abovementioned U.S. Patents is shown and labeled as xe2x80x9cPrior Artxe2x80x9d in FIG. 1 of the accompanying drawings. In this apparatus, there is provided a seal unit generally designated as 10 located at each end of a tubular metal part 12 to be hydroformed in a die set 14 comprising an upper die 16 and lower die 18 that cooperatively define a die cavity 20 about the part. Only one such seal unit is shown and it will be understood that a like seal unit is located at the other end of the part.
Each seal unit includes a hydraulically operated seal unit actuating mechanism comprising a hydraulic cylinder 22 in which a piston 24 is received and has a piston rod 26 projecting outward of the cylinder. A rod 28 commonly called a docking rod is connected at one end to the projecting end of the piston rod 26 and is adapted at the opposite end to dock with and sealing engage the respective end of the tubular part as shown. This occurs when hydraulic fluid at a predetermined pressure is delivered through a port 30 to a chamber 32 at one end of the piston while a chamber 34 at the other end of the piston is exhausted of hydraulic fluid through a port 36.
Still referring to FIG. 1, hydroforming fluid is then supplied to fill the thus sealed tubular part through a passage 38 in the docking rod and with the hydraulic pressure on the piston 24 sufficient at this stage of operation to maintain the sealing to prevent leakage of the hydroforming fluid. When the tubular part is filled with hydroforning fluid, the pressure on this fluid is then increased causing the tubular part to expand and conform to the die cavity surface stretching from its initial shape shown in solid lines to the expanded shape shown in phantom lines. At the same time, the hydraulic pressure in chamber 32 acting on the piston 24 is increased to maintain sealing of the hydroforming fluid in the part. And as the tubular part expands, its wall thickness will begin to thin since a fixed amount of material (metal) must now stretch to a new larger dimension.
To reduce and possibly eliminate such wall thinning, the hydraulic pressure in chamber 32 acting on the piston 24 is increased so that sufficient force is developed on the docking rod exceeding the yield strength of the tubular part. And this forces the tubular part to shorten thereby causing metal flow to the expanding tube portion to prevent or minimize its thinning. Following hydroforming of the part, the hydroforming fluid is exhausted through the passage 38 in each seal unit and the seal units are then separated or un-docked from the part by exhausting the chamber 32 and supplying the other chamber 34 in each seal unit with hydraulic pressure to retract their docking rod. After which, the die set is opened to remove the hydroformed part.
The apparatus in FIG. 1 does however require a very large hydraulically operated seal actuating mechanism for each seal unit because it must not only be required to develop a force exceeding the yield strength of the part to be hydroformed, it must also have the ability to overpower the high pressure of the hydroforming fluid in the part tending to un-dock or separate the seal units from the part during the hydroforming operation. And this pressure can reach 25,000 psi and more depending on the part being formed. In addition, there are safety standard limitations on the amount of hydraulic pressure that can be employed in a manufacturing facility. For example, the hydraulic pressure may be limited to 3000 psi for safety reasons. These standards may be self imposed by the manufacturer or government mandated but in either event, they can require increasing the size of the hydraulically operated seal actuating mechanism far beyond what would be required if the hydraulic pressure only had to produce sufficient force to adequately exceed the yield strength of the part being hydroformed.
Large hydraulically operated actuating mechanisms (hydraulic cylinder and piston) are very expensive and possibly even more important, they may not fit the available space in existing hydroforming apparatus and therefore require totally new apparatus to accommodate their large size.
The present invention in method and apparatus for hydroforming a tubular part provides for minimizing the size of the hydraulically operated seal actuating mechanism required to exceed the yield strength of the tubular part for the purpose of minimizing or prevent thinning of the part as it is hydroformed. This is accomplished in a very simple, low-cost manner with hydroforming apparatus comprising a pair of like seal units which are positioned in conventional manner at the opposite ends of the tubular part to be hydroformed. Each seal unit comprises a hydraulic cylinder, a doubleended hydraulic piston received in the hydraulic cylinder, a hydraulic chamber at each end of the hydraulic piston, a docking rod cylinder, and a docking rod that is located centrally of and rigidly joined at an intermediate axial location to the hydraulic piston.
The docking rod is received at one end in the docking rod cylinder and at the other end projects outwardly of the seal unit and is adapted at the latter end to dock with and sealingly engage the respective end of the tubular part to be hydroformed. And a hydroforming fluid passage is provided in each seal unit that is open to a fluid chamber in the docking rod cylinder at the one end of the docking rod and extends centrally through the docking rod to deliver hydroforming fluid to both this fluid chamber and the interior of the tubular part following the docking of the seal units with the part. With such docking being effected by the supply of hydraulic fluid under pressure to one of the hydraulic chambers while the other hydraulic chamber is exhausted.
The one end of the docking rod exposed to the hydroforming pressure in the fluid chamber in the docking rod cylinder has a pressure responsive area at least equal to or greater than that of the sealing end of the docking rod end acted on by the hydroforming fluid pressure in the tubular part that is forcing the latter to expand and conform to the die cavity. As a result, the hydroforming fluid pressure force acting outward on the seal units is counterbalanced or can even be overpowered by the hydroforming pressure acting in the opposite direction in the seal units on their docking rod as the latter pressure is raised to form the part. And as the hydraulic pressure acting on the hydraulic piston in the seal units is raised to yield or compress the part between its ends to minimize or prevent thinning of the part.
And thus the hydraulic piston in the seal units, by being relieved of having to counteract the hydroforming pressure force acting outward on the seal units, only needs to develop sufficient force in order to yield the tubular part to cause its shortening and thereby add material to the wall of the part being stretched to minimize or prevent its thinning. As a result, the hydraulic cylinder and piston for each seal unit can be considerably smaller than what would be required without the counterbalancing or overpowering hydroforming pressure force compensating feature described above.
It is therefore an object of the present invention to provide new and improved method and apparatus for minimizing the hydraulic force required to yield a part during its hydroforming to minimize or prevent thinning of the wall of the part as it is stretched.
Another object of the present invention is to provide a new and improved, simple, compact, low cost hydroforming method and apparatus by counterbalancing the force from the hydroforming pressure in the part acting outward on the seal units to minimize the size of the hydraulically operated mechanism required to yield the tube to prevent or minimize thinning of the wall of the part during hydroforming.
These and other objects, advantages and features of the present invention will become more apparent from the following description and accompanying drawings.